Closing the Circle: Returning Used Uranium to the Geologic Environment

Abstract

The used uranium oxide fuel discharged from CANDU nuclear power reactors has physical and chemical properties similar to those of natural uranium oxide found in uranium ore bodies in the earth's crust. When discharged from the reactor, less than 1% of the initial uranium atoms have been transmuted to new radioactive atoms. The long-term hazard is associated with four exceptionally long-lived radionuclides: iodine-129, cesium-135, technetium-99 and plutonium-239. These long-lived radionuclides can do harm only if they are ingested. The only practical way for this to occur is for the uranium oxide grains to be dissolved in groundwater. Since 1978, AECL has been developing the technology to dispose of used fuel in a vault excavated, 500 m to 1,000 m beneath the surface, in plutonic rock of the Precambrian Canadian Shield. Used-fuel bundles would be sealed in corrosion-resistant containers which would be placed in clay-lined holes bored in the floor of the vault. When filled with used-fuel containers, the vault would be backfilled and sealed with a mixture of clay and crushed rock. Compelling evidence for the safety of the disposal concept is provided from recent studies of uranium ore bodies in the earth's crust, most notably at Cigar lake. The uranium-ore body at Cigar Lake was formed about 1.3 billion years ago and, in spite of continuous contact with groundwater over this period, there has been no significant movement of uranium. Our understanding of the hydrogeology and geochemistry associated with the deposit gives us confidence that used fuel can be put back into the geological environment and isolated for its hazardous lifetime.